Instrumentation & Power Electronics

Instrumentation & Power Electronics Lecture 11 & 12 Introduction to Power Electronics

What is power electronics? 1) Definition Power Electronics:is the electronics applied to conversion and control of electric power.

What is power electronics? A more exact explanation: The primary task of power electronics is to process and control the flow of electric energy by supplying voltages and currents in a form that is optimally suited for user loads.

Power Electronic Devices • The power Electronic devices provides the utility of switching. • The flow of power through these devices can be controlled via small currents. • Power electronics devices differ from ordinary electronics devices in terms of their characterittics.

Power Electronic Devices • Power Semiconductor Devices can be classified into three groups according to their degree of controllability. • Diodes (on and off controlled by power circuit) • Thyristors (latched on by control signal but must be turned off by power circuit) • Controllable Switches (turned on and off by control signal)

Diodes • When the diode is forward biased it begins to conduct with only a small voltage across it. • When the diode is reversed biased only a negligibly small leakage current flow through the device until the reverse breakdown voltage is reached. • In normal operation reverse bias voltage should not reach the breakdown rating.

Diodes • Following figure shows the i-v characteristics of the diode.

Diodes • In view of very small current in blocking state and small voltage in conducting state , the i-v characteristics of the diode can be idealized.

Diodes • At Turn on, the diode can be considered as an ideal switch. It turns on rapidly compared to the transients in the power circuit. • However, at turn off, the diode current reverses for a reverse recovery time as shown in following figure. • The reverse recovery current can lead to overvoltage in inductive circuits.

Types of Diodes • Depending on the application requirement various types of diodes are available. • Schottky Diode • Fast Recovery Diode • Line Frequency Diode

Types of Diodes • Schottky Diode • These diodes are used where a low forward voltage drop (typically 0.3 v) is needed. • These diodes are limited in their blocking voltage capabilities to 50v- 100v.

Types of Diodes • Fast Recovery Diode • These diodes are designed to be used in high frequency circuits in combination with controllable switches where a small reverse recovery time is needed. • At power levels of several hundred volts and several hundred amperes such diodes have trr rating of less than few microseconds.

Types of Diodes • Line Frequency Diode • The on state of these diodes is designed to be as low as possible. • As a consequence they have large trr, which are acceptable for line frequency applications.

Diode With RC Load • Following Figure shows a diode with RC load. • When switch S1 is closed at t=0, the charging current that flows through the capacitor is found from

Diode With RL Load • Following Figure shows a diode with RL load. • When switch S1 is closed at t=0, the current through the inductor is increased

Diode With RL Load • Following Figure shows a diode with RL load. • When switch S1 is closed at t=0, the current through the inductor is increased.

Diode With RL Load • The waveform shows when t>>T, the voltage across inductor tends to be zero and its current reaches maximum value. • If an attempt is made to openS1 energy stored in inductor (=0.5Li2) will be transformed into high reverse voltage across diode and switch.

Example#1 • A diode circuit is shown in figure, with R=44Ω and C=0.1μF. The capacitor has an initial voltage Vo=220 v. If S1 is closed at t=0 determine: • Peak Diode Current • Energy Dissipated in resistor • Capacitor voltage at t=2μs

Example#1 • A diode circuit is shown in figure, with R=44Ω and C=0.1μF. The capacitor has an initial voltage Vo=220 v. If S1 is closed at t=0 determine: • Peak Diode Current

Example#1 • A diode circuit is shown in figure, with R=44Ω and C=0.1μF. The capacitor has an initial voltage Vo=220 v. If S1 is closed at t=0 determine: • Energy Dissipated in resistor • Capacitor voltage at t=2μs

Freewheeling Diode • If switch S1 is closed a current is established through the load, and then, if the switch is open, a path must be provided for the current in the inductive load. • This is normally done by connecting a diode Dm, called a freewheeling diode.

Freewheeling Diode • The circuit operation is divided into two modes. • Mode 1 begins when the switched is closed. • During this mode the current voltage relation is

Freewheeling Diode • Mode 2 starts when the S1 is opened and the load current starts to flow through Dm.

Freewheeling Diode • The waveform of the entire operation is given below.

Line Frequency Diode Rectifier • In most power Electronic systems, the power input is in the form of a 50Hz or 60Hz sine wave ac voltage. • The general trend is to use inexpensive diode rectifiers to convert ac into dc in an uncontrolled manner.

Single Phase Half Wave Rectifier • A single Phase half wave rectifier is the simplest type and is not normally used in industrial applications.

Single Phase Half Wave Rectifier • Although output voltage is D.C, it is discontinuous and contains Harmonics.

Single Phase Full Wave Rectifier • Each half of the transformer with its associated acts as a half wave rectifier.

Single Phase Full Wave Rectifier • Instead of using centre-tapped transformer we could use four diodes.

Three Phase Bridge Rectifier • Three Phase bridge rectifier is very common in high power applications. • It can operate with or without transformer and give six-pulse ripple on the out.

Three Phase Bridge Rectifier

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Instrumentation & Power Electronics